/* 
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

using DocIdSetIterator = Lucene.Net.Search.DocIdSetIterator;

namespace Lucene.Net.Util
{
	
	/// <summary>An iterator to iterate over set bits in an OpenBitSet.
	/// This is faster than nextSetBit() for iterating over the complete set of bits,
	/// especially when the density of the bits set is high.
	/// 
	/// </summary>
	/// <version>  $Id$
	/// </version>
	public class OpenBitSetIterator:DocIdSetIterator
	{
		
		// The General Idea: instead of having an array per byte that has
		// the offsets of the next set bit, that array could be
		// packed inside a 32 bit integer (8 4 bit numbers).  That
		// should be faster than accessing an array for each index, and
		// the total array size is kept smaller (256*sizeof(int))=1K
        // NOTE: Removed protected access for CLS-Compliance
	    /*protected*/ internal static readonly uint[] bitlist = new uint[]
	                                                            {
	                                                                0x0, 0x1, 0x2, 0x21, 0x3, 0x31, 0x32, 0x321, 0x4, 0x41,
	                                                                0x42, 0x421, 0x43, 0x431, 0x432, 0x4321, 0x5, 0x51,
	                                                                0x52, 0x521, 0x53, 0x531, 0x532, 0x5321, 0x54, 0x541,
	                                                                0x542, 0x5421, 0x543, 0x5431, 0x5432, 0x54321, 0x6,
	                                                                0x61, 0x62, 0x621, 0x63, 0x631, 0x632, 0x6321, 0x64,
	                                                                0x641, 0x642, 0x6421, 0x643, 0x6431, 0x6432, 0x64321,
	                                                                0x65, 0x651, 0x652, 0x6521, 0x653, 0x6531, 0x6532,
	                                                                0x65321, 0x654, 0x6541, 0x6542, 0x65421, 0x6543,
	                                                                0x65431, 0x65432, 0x654321, 0x7, 0x71, 0x72, 0x721,
	                                                                0x73, 0x731, 0x732, 0x7321, 0x74, 0x741, 0x742, 0x7421,
	                                                                0x743, 0x7431, 0x7432, 0x74321, 0x75, 0x751, 0x752,
	                                                                0x7521, 0x753, 0x7531, 0x7532, 0x75321, 0x754, 0x7541,
	                                                                0x7542, 0x75421, 0x7543, 0x75431, 0x75432, 0x754321,
	                                                                0x76, 0x761, 0x762, 0x7621, 0x763, 0x7631, 0x7632,
	                                                                0x76321, 0x764, 0x7641, 0x7642, 0x76421, 0x7643,
	                                                                0x76431, 0x76432, 0x764321, 0x765, 0x7651, 0x7652,
	                                                                0x76521, 0x7653, 0x76531, 0x76532, 0x765321, 0x7654,
	                                                                0x76541, 0x76542, 0x765421, 0x76543, 0x765431, 0x765432
	                                                                , 0x7654321, 0x8, 0x81, 0x82, 0x821, 0x83, 0x831, 0x832
	                                                                , 0x8321, 0x84, 0x841, 0x842, 0x8421, 0x843, 0x8431,
	                                                                0x8432, 0x84321, 0x85, 0x851, 0x852, 0x8521, 0x853,
	                                                                0x8531, 0x8532, 0x85321, 0x854, 0x8541, 0x8542, 0x85421
	                                                                , 0x8543, 0x85431, 0x85432, 0x854321, 0x86, 0x861,
	                                                                0x862, 0x8621, 0x863, 0x8631, 0x8632, 0x86321, 0x864,
	                                                                0x8641, 0x8642, 0x86421, 0x8643, 0x86431, 0x86432,
	                                                                0x864321, 0x865, 0x8651, 0x8652, 0x86521, 0x8653,
	                                                                0x86531, 0x86532, 0x865321, 0x8654, 0x86541, 0x86542,
	                                                                0x865421, 0x86543, 0x865431, 0x865432, 0x8654321, 0x87,
	                                                                0x871, 0x872, 0x8721, 0x873, 0x8731, 0x8732, 0x87321,
	                                                                0x874, 0x8741, 0x8742, 0x87421, 0x8743, 0x87431,
	                                                                0x87432, 0x874321, 0x875, 0x8751, 0x8752, 0x87521,
	                                                                0x8753, 0x87531, 0x87532, 0x875321, 0x8754, 0x87541,
	                                                                0x87542, 0x875421, 0x87543, 0x875431, 0x875432,
	                                                                0x8754321, 0x876, 0x8761, 0x8762, 0x87621, 0x8763,
	                                                                0x87631, 0x87632, 0x876321, 0x8764, 0x87641, 0x87642,
	                                                                0x876421, 0x87643, 0x876431, 0x876432, 0x8764321,
	                                                                0x8765, 0x87651, 0x87652, 0x876521, 0x87653, 0x876531,
	                                                                0x876532, 0x8765321, 0x87654,
	                                                                0x876541, 0x876542, 0x8765421, 0x876543, 0x8765431,
	                                                                0x8765432, 0x87654321
	                                                            };
		/// <summary>** the python code that generated bitlist
		/// def bits2int(val):
		/// arr=0
		/// for shift in range(8,0,-1):
		/// if val &amp; 0x80:
        /// arr = (arr &lt;&lt; 4) | shift
        /// val = val &lt;&lt; 1
		/// return arr
		/// def int_table():
		/// tbl = [ hex(bits2int(val)).strip('L') for val in range(256) ]
		/// return ','.join(tbl)
		/// ****
		/// </summary>
		
		// hmmm, what about an iterator that finds zeros though,
		// or a reverse iterator... should they be separate categores
		// for efficiency, or have a common root interface?  (or
		// maybe both?  could ask for a SetBitsIterator, etc...
		
		private readonly long[] arr;
		private readonly int words;
		private int i = - 1;
		private long word;
		private int wordShift;
		private int indexArray;
		private int curDocId = - 1;
		
		public OpenBitSetIterator(OpenBitSet obs):this(obs.Bits, obs.NumWords)
		{
		}
		
		public OpenBitSetIterator(long[] bits, int numWords)
		{
			arr = bits;
			words = numWords;
		}
		
		// 64 bit shifts
		private void  Shift()
		{
			if ((int) word == 0)
			{
				wordShift += 32; word = (long) ((ulong) word >> 32);
			}
			if ((word & 0x0000FFFF) == 0)
			{
				wordShift += 16; word = (long) ((ulong) word >> 16);
			}
			if ((word & 0x000000FF) == 0)
			{
				wordShift += 8; word = (long) ((ulong) word >> 8);
			}
			indexArray = (int) bitlist[word & 0xff];
		}
		
		/*/// <summary>** alternate shift implementations
		/// // 32 bit shifts, but a long shift needed at the end
		/// private void shift2() {
		/// int y = (int)word;
		/// if (y==0) {wordShift +=32; y = (int)(word >>>32); }
		/// if ((y & 0x0000FFFF) == 0) { wordShift +=16; y>>>=16; }
		/// if ((y & 0x000000FF) == 0) { wordShift +=8; y>>>=8; }
		/// indexArray = bitlist[y & 0xff];
		/// word >>>= (wordShift +1);
		/// }
		/// private void shift3() {
		/// int lower = (int)word;
		/// int lowByte = lower & 0xff;
		/// if (lowByte != 0) {
		/// indexArray=bitlist[lowByte];
		/// return;
		/// }
		/// shift();
		/// }
		/// ****
		/// </summary>*/
		
		public override int NextDoc()
		{
			if (indexArray == 0)
			{
				if (word != 0)
				{
					word = (long) ((ulong) word >> 8);
					wordShift += 8;
				}
				
				while (word == 0)
				{
					if (++i >= words)
					{
						return curDocId = NO_MORE_DOCS;
					}
					word = arr[i];
					wordShift = - 1; // loop invariant code motion should move this
				}
				
				// after the first time, should I go with a linear search, or
				// stick with the binary search in shift?
				Shift();
			}
			
			int bitIndex = (indexArray & 0x0f) + wordShift;
			indexArray = (int) ((uint) indexArray >> 4);
			// should i<<6 be cached as a separate variable?
			// it would only save one cycle in the best circumstances.
			return curDocId = (i << 6) + bitIndex;
		}
		
		public override int Advance(int target)
		{
			indexArray = 0;
			i = target >> 6;
			if (i >= words)
			{
				word = 0; // setup so next() will also return -1
				return curDocId = NO_MORE_DOCS;
			}
			wordShift = target & 0x3f;
			word = (long) ((ulong) arr[i] >> wordShift);
			if (word != 0)
			{
				wordShift--; // compensate for 1 based arrIndex
			}
			else
			{
				while (word == 0)
				{
					if (++i >= words)
					{
						return curDocId = NO_MORE_DOCS;
					}
					word = arr[i];
				}
				wordShift = - 1;
			}
			
			Shift();
			
			int bitIndex = (indexArray & 0x0f) + wordShift;
			indexArray = (int) ((uint) indexArray >> 4);
			// should i<<6 be cached as a separate variable?
			// it would only save one cycle in the best circumstances.
			return curDocId = (i << 6) + bitIndex;
		}
		
		public override int DocID()
		{
			return curDocId;
		}
	}
}